The present invention relates to a fuel injection pump. More particularly, it relates to a fuel injection pump with adjustment of a supply start and thereby adjustment of an injection start.
An adjustment of the injection start in fuel injection arrangements serves for compensation of time delays which take place during injection process so as to change the supply start of the pump piston relative to respective rotary position of the pump drive. The time delays take place between the pressure increase in the pump and the injection start as well as between the injection start and the combustion start. The pressure transmission from the pump to a nozzle is influenced by the compressibility of the fuel, the running time of the pressure shaft, the pressure increase time to attainment of the opening pressure of the nozzle, and the supply time required for compensation of the unloading volume. Between injection and ignition of the fuel, a so-called ignition delay takes place because of reaction-kinetic grounds. The ignition delay measured in degrees of the crank shaft increases with the number of revolutions and changes simultaneously with the injection start.
In the fuel injection pump of the type to which the present invention belongs, namely with a pump piston drive provided with adjusting means, the injection adjustment is performed in dependence on the number of revolutions. The greater part of the working power of the injection adjustor is used for the compensation of the time delays. The remaining part of the working power serves, depending on the requirements to the size of the internal combustion engine, for improvement of the fuel consumption, the efficiency, the movement hardness and/or the exhaust gases.
In a known fuel injection pump of this type disclosed for example in the U.S. Pat. No. 4,168,940, an adjusting piston for adjusting the pump piston is displaced in the region of idle number of revolutions and low numbers of revolutions against the force of a first spring, till after covering of a predetermined adjusting path, in the medium number of revolutions region a second spring is brought in engagement to provide action in the same direction. Starting from this point, the characteristic line of the injection adjustor extends flatter, whereby first of all the movement hardness is improved without worsening the exhaust gas value. The injection start can thereby be adjusted to the exhaust gas possibilities. The disadvantage of this known arrangement resides in the difficulty because of the outer engagement of the number of revolutions region from which the second spring arrives into engagement. A calibration can be obtained extremely difficult, mainly by dismounting of the injection adjustment and insertion of spacer discs.
A further disadvantage of this known arrangement is its relatively expensive construction. The second spring which normally is substantially weaker than the first continuously operating spring must cooperate with a driving device which is brought in engagement from the region of a medium numbers of revolution. For this purpose a sleeve is provided which is axially displaceable on a housing-fixed pin, and the second spring is clamped between the sleeve and the housing. Starting from a certain number of revolutions and displacement of the injection adjusting piston, this sleeve is driven by the adjusting piston so that the spring is compressed. The thus designed relatively expensive construction brings about the danger of high failure susceptibility.
Since there is here a so-called hydraulic injection adjustor with which the adjusting piston is displaced because of hydraulic forces increasing with the number of revolutions, the characteristic line of this injection adjustor is naturally also changed under the action of the hydraulic pressure. All known solutions are substantially expensive and have the disadvantage of high temperature dependence. It is known that the diesel fuel is considerably more viscous at low temperatures than at high temperatures so that the distortion of the characteristic especially with cold pump takes place.